The present invention relates to a display apparatus.
Some display apparatuses have view angle dependence in display performance. Liquid crystal display (LCD) apparatuses represented by a twisted nematic (TN) LCD apparatus are known as typical examples of such display apparatuses. The view angle dependence (or viewing angle characteristic) of display performance as used herein refers to the phenomenon that the display performance such as the contrast ratio, the gray scale characteristic and the hue is different between when the display is viewed from the front (in the direction normal to the display plane of the display apparatus) and when it is viewed in an oblique direction. In general, the display performance is lower when the display is viewed in an oblique direction than when it is viewed from the front.
As far as the LCD apparatus is concerned, the view angle dependence of display performance occurs because of use of liquid crystal molecules having refractive index anisotropy and members such as polarizing plates. This is therefore a problem related to the nature of the LCD apparatus. Conventionally, various methods have been proposed and developed to solve the problem of the view angle dependence. These methods include: a halftone gray scale method in which one pixel is divided into a plurality of regions and voltages different at a fixed ratio are applied to these regions; a domain division method in which one pixel is divided into a plurality of domains having liquid crystal molecules rising in different directions; an in-plane switching (IPS) method in which a transverse electric field is applied to liquid crystal molecules; a multi-domain vertical alignment (MVA) method in which vertically-aligned liquid crystal molecules are driven; and an optically compensated birefringence (OCB) method in which a bend-aligned cell and an optical compensation film are combined. In these methods, however, the structures of alignment films, electrodes and the like must be changed. For this change, it is necessary to establish a fabrication technology and provide a new fabrication facility, and this causes difficulty in fabrication and cost increase.
There is known a method permitting improvement of the problem of the view angle dependence without the necessity of changing the structures of an alignment film, electrodes and the like at all, in which an optical compensation film is incorporated in a conventional TN mode LCD apparatus. For example, JP 8-50206A proposes a method using an optical compensation film including discotic liquid crystal molecules. In a normally-while TN mode LCD apparatus, liquid crystal molecules are mostly aligned vertically with respect to electrode substrates during application of a voltage. Near the substrates, however, the liquid crystal molecules are aligned in parallel with the substrates due to a strong anchoring effect of the substrates. Therefore, the directors of the liquid crystal molecules are not uniform but gradually change in the thickness direction, forming a hybrid structure. In view of this fact, based on the idea of canceling the refractive index anisotropy of the aligned state of the liquid crystal molecules in the liquid crystal cell observed during voltage application, a total of two optical compensation films including a hybrid-aligned discotic liquid crystal material are interposed between a liquid crystal panel and polarizing plates placed on both surfaces of the liquid crystal panel, to thereby improve the view angle dependence.
Other methods based on combinations of various modes and various optical compensation films have also been proposed and developed for solving the problem of the view angle dependence of display performance. These methods have their own effects and can improve the problem of the view angle dependence of display performance to some extent. However, the effects of these methods have only been verified by a measurement scheme in which the expanse of a view angle range within which the contrast ratio is greater than a predetermined value is measured as the viewing angle, or a measurement scheme in which the expanse of a view angle range within which no gray scale inversion occurs and the contrast ratio is greater than a predetermined value is measured as the viewing angle. To state specifically, in examples in JP 2002-156527A, change of the luminance of each of gray scale levels L1 to L8 with change of the view angle is examined, in which L1 denotes the black display state, L8 denotes the white display state and L2 to L7 denote intervening halftone display states roughly equally spaced between the black and white display states. The view angle range within which the contrast ratio represented by the ratio of L8 to L1 is 10 or more and no gray scale inversion is observed between L1 and L2 is determined as the viewing angle for the four directions, upward, downward, leftward and rightward directions. The effect of improving the problem of the view angle dependence is verified by examining whether or not the determined viewing angle is wider than a comparative example.
In other words, in the measurement scheme described above, only the level of the contrast ratio and the presence/absence of gray scale inversion are considered as to the viewability in an oblique direction. No consideration is made at all on the influence of the view angle dependence of the gray scale characteristic on the display performance. The gray scale characteristic as used herein refers to the relationship between an input signal and its output luminance of a display apparatus, which is generally called a γ characteristic. The γ characteristic is an important characteristic in attaining reproduced output luminance proportional to the luminance of an object, and is especially related to the naturalness of display. If this characteristic is poor, so-called white saturation and black saturation occur in a bright portion and a dark portion. Therefore, in the scheme described in JP 2002-156527A, in which a view angle range within which no gray scale inversion is observed between L1 and L2 and the contrast ratio is 10 or more is measured and determined as the viewing angle for the four directions, upward, downward, leftward and rightward directions, if the γ characteristic obtained when the display is viewed at a certain view angle is greatly different from that obtained when it is viewed from the front, the display will be unnatural and the viewability will be low even when the view angle is within the view angle range described above.